Radio frequency identification (RFID) is an identification technology utilizing radio frequency (RF) signals to transmit and receive information in a wireless manner. Energy transfer occurs between the RFID communications. Therefore, RFID tag and RFID reader can exchange data without physical contact. Further, no additional power is needed for the RFID tag.
RFID system principally includes a tag and a reader. The tag implemented by a mini integrated circuit (IC) chip is embedded in or attached to an object. Then, the reader can read information stored in the IC chip by RFID technology.
For the great demand for higher data rates, longer accessible distances, and smaller antenna's size of the reader, there is a tendency towards higher-frequency wireless signal application. It is anticipated that ultrahigh frequency (UHF) band (860˜960 MHz) and even microwave band (2.4˜2.5 GHz) RFID system will take the place of the relative lower band (125 kHz and 13.56 MHz) RFID system in the near future.
For reading different tags operating at different frequency bands, a reader compatible with dual band is required. FIG. 1 is a block diagram exemplifying a frequency synthesizer conventionally used in a dual-band RFID reader. The frequency synthesizer 10 includes a phase-locked loop (PLL) module 101, a first frequency divider 102 and a second frequency divider 103. The PLL module 101 includes a phase/frequency detector (PFD) 1010, a charge pump 1011, a loop filter 1012, a first voltage controlled oscillator (VCO) 1013 and a second VCO 1014. In one example, the first VCO 1013 and the second VCO 1014 are selected to output first clock signals with frequency range from 1720 MHz to 1920 MHz and second clock signals with frequency range from 4.8 GHz to 5 GHz, respectively. After a divide-by-2 operation by corresponding frequency divider 102 or 103, the output clock signals are converted into third clock signals with frequency range from 860 MHz to 960 MHz or fourth clock signals with frequency range from 2.4 GHz to 2.5 GHz. Hence, the dual-band RFID reader using the frequency synthesizer 10 can reads both UHF band tags and microwave band tags. The plural VCOs, however, complicates the circuitry, occupies considerable chip area and consumes power. A frequency synthesizer with a simplified architecture is desired.